In an evaporative fuel purging apparatuses used for an internal combustion engine in which an evaporative fuel generated in an fuel tank is absorbed by an absorbing agent of a canister; the evaporative fuel thus absorbed is released from the absorbing agent with the intake of the air; and the evaporative fuel is supplied (purged) to an intake system through an purging passage, abnormalities of the purging system can be diagnosed by such a process as follows.
After the pressure-reduction treatment for the purge system, both a drain shut-off valve provided on a drain port for sucking the atmospheric air in a canister and a purge cut valve provided in a purge passage are closed to form an enclosed space, and thereby the abnormality diagnosis for the purge system is performed by checking the leak down. Namely, when there occurs a leakage due to gaps generated in the piping connections, valves, seals or the like in the closed purge system, the tank internal pressure is greatly increased, which is diagnosed to be abnormal.
After termination of such an abnormality diagnosis for the purge system described above, if the purge cut valve is opened while closing the drain shut-off valve or simultaneously opening the same to perform the purge control of the evaporative fuel again, the fuel vapor with high concentration, which has remained during leak down check in the canister and the purge tube which are pressure-reduced in a state where the evaporative fuel is liable to be generated, is purged in the intake system at a stretch. This causes the rapid change in the air-fuel ratio, thereby bringing the deterioration in the emission and drivability.
Taking the above circumstances into consideration, the present invention is made, and its object is to provide an evaporative fuel control unit used for an internal combustion engine capable of preventing the rapid change in the air-fuel ratio after abnormality diagnosis for the purge system, thereby securing the stabilization of the emission and the drivability.